1. Field of the Invention
The present invention relates generally to scroll saws or jig saws and, more particularly, relates to scroll saws or jig saws having a saw blade that is connected between the free ends of a generally C-shaped or U-shaped yoke which is driven to reciprocate at a pivot point located at the yoke's rear or center. The present invention also relates generally to apparatus for tensioning a saw blade between the free ends of a generally C-shaped or U-shaped yoke typically incorporated in a scroll saw or jig saw.
The terms scroll saw and jig saw will be used interchangeably in the following description of the invention and it is intended that either term refers generally to any device wherein an elongate saw blade (i.e., a saw blade having a length significantly greater than a width) is connected between the free ends of a yoke which is driven to reciprocate at a pivot point located generally at the yoke's rear or center. Also, the terms C-arm and yoke are used interchangeably in the following description of the invention to refer to the generally C-shaped or U-shaped member typically incorporated in a scroll saw or jig saw to retain a saw blade.
2. Description of the Invention Background
Scroll saws and jig saws typically include a substantially C-shaped or U-shaped member or yoke that reciprocates around a pivot point at the yoke's rear or center. The substantially C-shaped or U-shaped yoke comprises two substantially parallel arms connected by a bight portion. A saw blade is connected between the free ends of the two arms and is driven by a motor to reciprocate to thereby cut a workpiece. The yoke must transfer the force generated by the motor to the saw blade and must be sufficiently stiff to maintain proper tension on the saw blade. The yoke must also withstand heavy cyclic stresses, especially when the machine is under heavy load. Accordingly, yokes incorporated in prior art scroll saws and jig saws have been manufactured of a metal such as steel or aluminum and are, therefore, quite heavy. As an example, the metal two-armed C-shaped yoke of a typical 18" scroll saw may weigh three pounds or more.
The metal yoke's significant weight places a significant load on the bracket and bearing members about which the yoke reciprocates and may contribute to premature failure of those members. The significant weight of a metal yoke also produces a large amount of vibration during reciprocation. The vibration may reduce the cutting precision of the machine and make intricate cuts impossible. To counter vibrations, almost all prior art scroll saws or jig saws incorporate a counter-rotating balancing mechanism at the reciprocating bearings coupling the motor to the yoke. One example of such a counterbalance mechanism is provided in U.S. Pat. No. 4,949,616 to Chang wherein a first eccentric plate links the motor's revolving shaft and the lower arm of the yoke, and a second eccentric plate is provided opposite the first eccentric plate to neutralize vibrations during sawing.
The use of a counterbalance mechanism increase the cost of a scroll or jig saw and adds significant weight to the reciprocating bearings, thereby reducing the service life of the bearings. Counterbalance mechanisms also increase the complexity of the machine, and complicate and increase the cost of repairs.
Scroll saws and jig saws also include means to attach the ends of a saw blade to the free ends of the two-armed yoke. An end of a saw blade is typically attached to the free end of an arm of the yoke by a saw blade chuck which exerts a clamping force on the blade. In many blade chuck designs, the operator must hand-tighten a set screw or nut to directly exert clamping force on the blade. U.S. Pat. No. 5,363,733 to Baird et al. provides a quick-releasing blade chuck wherein a multiple of the force applied by the operator may be indirectly applied to clamp the end of the saw blade to the yoke.
It is quite difficult to adjust the tension on the saw blade using such blade chucks. If the blade is not correctly tensioned, it may break when it encounters a workpiece. An example of a prior art releasable blade tensioning apparatus is disclosed in U.S. Pat. No. 2,721,587 to Dremel wherein a jigsaw has a pivotable blade holder attached to the end of the upper arm which can be pivoted between blade tightening and blade releasing positions. In the blade tightening position, a lever can be pivoted into position to lock the blade carrier in place with the blade under tension. However, no means are provided to readily adjust the amount of tension that is applied to the blade.